DE AS 1 625 680 deals with a friction body for wet clutches and brakes, having a carrier and at least one sintered, porous and metallic friction lining fitted to the carrier. It was proposed that the friction lining consists of metal fibers, with the intention being for the degree of porosity to be at least 50%.
It is known from DE 10 2010 049 797 A1 that a brake disk can be produced integrally with a wheel hub, and therefore it should be possible for the axial run-out of the brake disk to be reduced. In addition, the friction surfaces of the brake disk could be provided with a friction coating, which can consist of a hard metal or of a ceramic.
EP 1 987 267 B1 deals with a brake disk based on the use of materials, one of which is to perform a structural function and the other is to perform a brake function. The brake disk comprises a support or structural disk, the sides of which are equipped with a first and a second friction disk. The friction disks are produced from a material suitable for performing the brake function. The structural disk is produced from composite material. The composite material of the structural disk can consist of a resin, optionally from among epoxy, phenolic, cyanoester, cyanoepoxy and ceramic resins and enamel or a combination thereof. The friction disks can be produced from a material selected from among steel, cast iron, aluminum alloy, aluminum oxide (ceramic), silicon carbide, silicon nitride, titanium carbide and carbon ceramic.
In vehicles, in particular in motor vehicles, disk brakes are by far the most common type of brake systems. Disk brakes are composed substantially of a brake disk and a brake caliper which surrounds the brake disk on the edge. Here, the brake disk is connected to the wheel of the vehicle to be braked by way of a wheel hub mounted rotatably in the steering knuckle. By contrast, the brake caliper is fixed to the steering knuckle. The actual deceleration is achieved by brake pads which can be placed against the brake disk and which are arranged on both sides of the brake disk between it and the brake caliper.
Depending on the application, brake disks can consist both of iron, e.g. of gray cast iron (GCI), but also of carbon ceramic or aluminum.
Brake disks are typically cast from unalloyed gray cast iron (GCI) material. Although disks of this type can be cast and machined cost-effectively, they do not afford adequate corrosion protection against spray water from the road surface or rain water. In winter in particular, an increased corrosive attack by road salt can be observed. If vehicles are then parked for a relatively long period of time, the severe corrosion on the gray cast iron surface in the region of the friction ring can have the effect that the brake lining rusts solid, so to speak, on the brake disk. This can be attributed to the fact that the lining in a brake disk permanently bears tightly against the disk surface and rust can therefore form in this narrow gap. When the vehicle is then moved again, lining material which has rusted up can be torn out of the lining and transferred onto the disk surface. This leads to juddering of the brakes in association with a high generation of noise. In certain circumstances, it may be necessary to change the disks or to remove the adhering rust by turning. Moreover, rusty brake disks appear to be substandard if the rusted disks can be seen through the aluminum rims of premium appearance.
Furthermore, it is known that the wear resistance of the GCI brake disks is not sufficiently high. The brake linings typically used are optimized for a particular coefficient of friction, and in this respect a certain loading of the friction ring with abrasive wear is accepted. This abrasive loading has the effect, inter alia, that brake disks with red rust formation after a rainy day are metallically blank again through the actuation of the brakes when the car is next driven.
The abrasion between the brake lining and the brake disk forms particle emissions, i.e. fine dust. In addition to the problem in relation to fine dust, the visual effect of rusted brake disks in combination with expensive, premium aluminum rims also plays an additional role. It is known that approximately 70% of the fine dust particles originate from the GCI disk material. These wear particles are at a very high temperature of approximately 700° C., at which they strike against the aluminum rim. In the process, they can easily burn into the clearcoat on the aluminum surface and it is very difficult to remove the gray-black coating even in the car washing plant and with intensive maintenance. Moreover, squeaking noises or the juddering of brakes in the case of linings which have rusted up after a relatively long standstill are additionally regarded as annoying.
Therefore, intensive development work is carried out worldwide in order to improve both the corrosion resistance and the wear resistance of the brake disks. In this respect, disks are produced from a high-grade steel casting material, for example. Although the problem relating to corrosion can be eliminated in this case, the wear resistance is improved only slightly. In addition to the high costs resulting from the use of strategically important elements such as chromium and nickel, the wear resistance is improved only slightly. In addition, the thermal conductivity is reduced considerably, as a result of which the wheel bearings may be subjected to greater thermal loading.
Furthermore, numerous galvanic coating processes have been proposed for solving the problem. The production of these layers is very complex: For this purpose, it is necessary for the entire component to be coated, for example with chromium or nickel or Ni plus hard material particles. It is often the case that layers of this nature also have to be anchored to the substrate material by a diffusion annealing treatment, in order to cope with the loading of the brakes.
Another possibility is represented by thermal spraying processes: For this purpose, the GCI disks are roughened by profile turning and subsequent corundum blasting and then provided, for example, with a 17% Cr steel sprayed layer having a thickness of 500 μm. Powder and wire spraying processes are used for this purpose. After coating, the rough sprayed layers have to be reworked by turning or grinding in order to comply with the required dimensions of the brake disks. In this sprayed coating, however, only the region of the friction surface is coated, and therefore the hub region has to be protected against the onset of rust, as is common at present, by varnishing. For ventilated brake disks, the spraying process can of course produce no corrosion protection for the region between the cover disks, and therefore the webs still tend to become rusty and rusty water will then run over the friction surface with the expensive sprayed layer. Furthermore, it is found in salt spray tests that thermally sprayed layers are infiltrated on account of the microporous structure, as a result of which undercorrosion can arise. This corrosion can be prevented only by expensive sealing processes. Merely the outlay for coating the GCI disks with, for example, a reground 17% Cr steel sprayed layer having a thickness of 500 μm is greater than the entire production to date for the same disk by casting plus mechanical reworking.
In the case of GCI disks or disks with a ground thermally sprayed layer, the brake lining has an abrasive effect on the surface and material is removed by abrasion upon every braking operation. Although the material removal is considerably less in the case of the hard sprayed layer, the abrasion mechanism is maintained.
It is also possible to provide what are termed temporary, cost-effective protective layers, so that the vehicles are at least transferred from the manufacturer to the end consumer without the brand-new vehicle already having rusty brake disks. These are usually colored sprayed layers containing zinc pigments. On the other hand, brake systems are known in which zinc is rubbed onto the GCI surface during the braking operation and cathodic corrosion protection arises as a result. On the other hand, this zinc film has a negative effect on the friction function of the brake lining, and the coefficients of friction fall. In this respect, the zinc protective layer is expediently a long way away from initial operation.
A nitriding diffusion coating on the basis of Fe nitride would also be possible. This coating leads to temporary protection against wear and corrosion, but the service life of this coating in fact appears to be limited to less than 40 000 km and also to be suitable only for NAO linings on the USA market. This is because at the speed limits which are much higher, and in some cases not present at all, in Germany, for example, higher brake temperatures are to be expected than, for example, in the USA, and therefore the NAO linings appear suitable there. Moreover, the process is very time-consuming and very expensive owing to the large furnace chamber required.
Numerous thermal spraying processes (these have already been mentioned above) and galvanic coating processes are similarly used. The production of these layers is very complex. In salt spray tests, however, both galvanic coatings of this type and thermally sprayed coatings perform rather poorly. Thus, the infiltration of thermally sprayed layers cannot be reliably avoided even with additional sealing processes.
In the light of the prior art indicated, the simple and sustainable production of brake disks as mass-produced items still affords room for improvement.